The coastline, or shoreline, is determined by averaging the high water marks left by the ebb tide. This average line serves as the accepted boundary for official mapping and charting purposes.
The Shifting Nature of Coastlines
Coastlines are not static lines but are constantly changing due to the influence of tides, waves, erosion, and deposition. This endless shift means that establishing a fixed line requires a specific method.
The Standard Method Based on High Water Marks
According to common charting practices, the method for determining the coastline relies on tidal patterns.
Here's a breakdown of the process:
- Tidal Influence: Each time the tide goes out (ebb tide), it leaves a mark indicating the highest point the water reached during that cycle. This is known as the high water line.
- Relentless Shift: Because tides vary daily and over longer cycles, these high water marks constantly change position.
- Averaging the Marks: To create a stable and usable boundary for maps and charts, these individual high water marks are averaged. This averaging process accounts for the variations over time.
- The Accepted Shoreline: The line resulting from the averaging of high water marks becomes the accepted shoreline used for navigation, mapping, and measurement purposes.
- Measuring Length: This accepted shoreline is the specific line that is measured when determining the length of a country's or region's coastline.
In essence, the coastline depicted on maps and charts is not the instantaneous edge of the water at any given moment, but rather a calculated average position derived from the highest reach of the tides over a period.
Why This Method?
Using the averaged high water mark provides a consistent and reproducible standard for charting and legal boundaries, despite the dynamic nature of the actual physical coastline. It's a practical solution for representing a constantly moving feature on static maps.
